U.S. Pat. No. 5,878,496 describes a method of manufacturing a component by superfinish hard machining wherein the component is formed, hardened by heat treatment, and then hard machined to achieve a surface finish that is comparable to abrasive-based superfinishing operations such as finish grinding, honing, lapping, polishing, electropolishing, and superfinishing while imparting residual stress to the hard machined component in a manner to optimize its service life, such as fatigue life of a hardened bearing race. The hard machining method of the patent involves machining the pre-hardened surface of the component by turning or other cutting operation to achieve the desired surface finish. For example, hard turning typically can achieve a surface finish of 32, preferably 16, microinches average surface roughness (Ra) or less on a hardened bearing steel component having a Rockwell hardness of Rc of 35 and higher, such as Rc of 60 or more while imparting a residual stress thereto that optimizes fatigue life. As described in the patent, practice of such hard machining eliminates the need for separately performed machining steps or operations that include rough machining prior to hardening and multiple grinding operations and abrasive-based superfinishing subsequent to hardening in manufacture of a bearing race.
The patent involves modeling the component by actual physical testing and/or computer software modeling thereof, applying in-service loading conditions to the model to determine the level and location of in-service stresses on the component, determining a critical in-service stress that affects service life, determining particular hard machining parameters to impart a particular residual stress to counter the critical in-service stress and a particular surface finish to the pre-hardened component, and then hard machining the pre-hardened surface of the component using the determined hard machining parameters to impart the particular residual stress and surface finish thereto to optimize service life of the machined component and reduce the number of machining operations needed to manufacture the component.
The determination of particular hard machining parameters is made by determining interelationships of machining parameters to residual stress and to surface finish. Such determinations are made by fabrication of a number of specimens using different combinations of machining parameters (e.g. cutting speed, feed rate, and depth of cut) and then modeling the interrelationship of the machining parameters with separate equations for surface finish and integrity to achieve residual stress. For example, an interrelationship of surface finish and machining parameters is determined using equation A of the patent, while an interrelationship of residual stress distribution and machining parameters is determined using equations B(1) and B(2) of the patent.
In the past in the manufacture of bearing races, each bearing race is formed and then annealed in preparation for rough turning in the relatively soft condition of the bearing material. The rough turned bearing race then is hardened by heat treatment and/or case hardening followed by double disk grinding, cylindrical grinding, centerless grinding and finally abrasive-based superfinishing. The expected fatigue life of bearing races made this way has been determined by industry by destructive testing under anticipated service loading conditions. Such destructive testing requires expensive equipment and labor, and extended testing time to determine fatigue life of such bearing races.